Postcode Formats

UK postcodes (originally named post codes) are alphanumeric in design, with each full postcode being known as a 'postcode unit'. Each postcode corresponds to a particular quantity of physical addresses or a single delivery point in the UK.

Each postcode consists of a series of numbers and letters, being between six and eight alphanumeric characters in length, and a single space that separates the 'outward' and 'inward' sections of the postcode. They can represent either a single address, part of a street, or in some cases a whole street.

Outward codes

Outward codes (e.g. SS3) identifies the geographical postcode area, and consists of one or two alphabetical characters, for example S for Sheffield, TQ for Torquay etc. Some outward sections of the postcode represent a large geographical area, for example RH, which covers the whole of north Sussex. These first characters are then followed by one or two numerical digits to identify the postcode district within the outward code area. For example, S1, PE1, and EC2M.

Inward codes

The inward part of the postcode identifies a smaller geographical area within the outward boundary. The first character (typically a number) identifies the postcode unit, which can be a single property or group of properties.

Postcode validation

Format	Example	Coverage
A9 9AA	S1 1AA	B, E, G, L, M, N, S, W postcode areas
A99 9AA	M60 1NW
AA9 9AA	CR2 6XH	All postcode areas except B, E, G, L, M, N, S, W, WC
AA99 9AA	DN55 1PT
A9A 9AA	W1A 1HQ	E1W, N1C, N1P, W1 postcode districts
AA9A 9AA	EC1M 1BB	WC postcode area; EC1-EC4, NW1W, SE1P, SW1 postcode districts

Additional notes to take into account for the formatting of UK postcodes:

1. Areas with only single-digit districts: BR, FY, HA, HD, HG, HR, HS, HX, JE, LD, SM, SR, WC, WN, ZE.
2. Areas with only double-digit districts: AB, LL, SO.
3. Areas with a district '0' (zero): BL, BS, CM, CR, FY, HA, PR, SL, SS (BS is the only area to have both a district 0 and a district 10).
4. The following central London single-digit districts have been further divided by inserting a letter after the digit and before the space: EC1-EC4 (but not EC50), SW1, W1, WC1, WC2, and part of E1 (E1W), N1 (N1C and N1P), NW1 (NW1W) and SE1 (SE1P).
5. The letters QVX are not used in the first position.
6. The letters IJZ are not used in the second position.
7. The only letters to appear in the third position are ABCDEFGHJKPSTUW when the structure starts with A9A.
8. The only letters to appear in the fourth position are ABEHMNPRVWXY when the structure starts with AA9A.
9. The final two letters do not use the letters CIKMOV, so as not to resemble digits or each other when hand-written.
10. Post code sectors use digits from 1 to 9 followed by 0 (the Royal Mail originally sorted sector 0 after 9, treating it as the 10th not the 1st sector label).

In total, there are 29 million postcodes in operation. Postcodes can be validated against the Royal Mail Postcode Address File (PAF), listing all 29m delivery postcodes that make up much of the UK.

Source: Wikipedia

British Forces Post Office

Providing a postal service to HM Forces, the British Forces Post Office (BFPO) delivers mail in the UK and to territories across the globe. Post sent to these locations are prefixed with BFPO, followed by a space and a number that can be 1-4 digits.